Fixed remaining SyntaxErrors

I know that werkzeug.script is deprecated anyway, but it still makes the
testsuite fail.
Some more fixes.

Since the builtin dictionary methods have changed their behavior, and
Werkzeug's datastructures inherit from them, their API also gets changes
in many places. For this i've created a mixin that redefines keys,
values, items and the iter* methods to behave just like in Python 2.
This seems to me the most reasonable approach.